1. Field of the Invention
This invention relates to methods and apparatus for the vapor condensation heating of articles to an elevated temperature and, more particularly, to controllably exposing only a selected underside surface area of an article to be heated to a generated body of hot saturated vapor of substantially uniform volume, while continuously minimizing any loss of the vapor to the atmosphere.
2. Description of the Prior Art
In the mass soldering, fusing or brazing of articles, conventional methods and apparatus, such as involving the use of a typical soldering iron, are generally not appropriate. More specifically, there has developed an urgent need in the electronics industry for methods and apparatus for performing mass (or selected) soldering (or unsoldering) operations on complex printed circuit boards which, for example, may require hundreds (or even thousands) of closely spaced connections to be soldered. It is also very desirous to be able to carry out such soldering operations in a manner that obviates the formation of surface oxidation in the absence of flux being applied to the solder areas.
Although the present invention is not to be construed as limited to a particular type of heat-induced operation, the nature thereof is most readily understood in the context of performing a soldering (or unsoldering) operation on an article, particularly on one surface thereof, such as on a selected underside surface of a printed circuit board, during the manufacture or repair thereof.
In a typical hand soldering operation, utilizing a soldering iron, as well as in a conventional automated wave soldering operation, a coating of flux has normally been required, and applied on at least the article areas to be soldered, in order to minimize any deleterious oxide surface build-up during soldering.
In order to overcome the troublesome manufacturing problems associated with oxide surface build-up, and the attendant need for flux, there has recently been increasing use made of vapor condensation soldering processes and apparatus. One such process and apparatus is the subject of U.S. Pat. No. 3,866,307 of R. C. Pfahl, Jr. et al, issued Feb. 8, 1975, assigned to the assignee of the present invention, and incorporated herein by reference.
In accordance with the teaching of the prior Pfahl et al. patent, the article to be soldered (fused or brazed) is placed within a vessel that is open to the atmosphere on the top side so as to facilitate the entry and removal of the article therefrom. Each article to be heated to a desired elevated temperature is immersed within a primary body of hot saturated vapor generated within the vessel, with a portion of the vapor body condensing on the article and transferring thereto its latent heat of vaporization. This heats the article to the temperature required to perform a soldering operation, for example, thereon. The hot saturated vapor body is generated by continuously boiling within the vessel a heat transfer liquid that is non-conducting, chemically inert, and has a boiling point at least equal to, but preferably above, the temperature required to melt the solder. Such a vapor condensation facility may also be employed to perform a mass re-flow soldering operation on a continuously moving line of articles.
The various preferred heat transfer liquids presently employed to heat articles in the manner described above, and which liquids are described in greated detail hereinbelow, are quite expensive. As such, any appreciable loss of the generated vapor in question to the atmosphere significantly impacts on the over-all costs incurred in carrying out a given soldering operation, particularly high volume mass soldering operations.
One technique utilized heretofore to at least partially minimize the loss of the relatively expensive primary vapor to the atmosphere in an open top vessel has has involved positioning a suitable cooling coil (or coils) about the inner stationary sidewalls of the vessel at an elevation near the top thereof. Such a cooling coil (or coils) establishes a so-called vapor barrier that condenses any vapor that rises to the elevation, and in the immediate vicinity, of the coils. This technique, however, is not always completely effective in condensing the major portion of the rising vapor in the central region of the vessel unless the established vapor barrier has appreciable depth. A vessel that incorporated both a peripherially disposed cooling coil and a completely enclosing, but removable, top wall or cover is disclosed in U.S. Pat. No. 4,022,371 of E. R. Skarvinko et al. Such an apparatus, of course, not only requires the total immersion of the articles within the vapor, but the removal and re-positioning of the cover from the vessel in connection with each heating operation, with the attendant loss of vapor to the atmosphere at such times.
A more effective technique recently developed to minimize the loss of the relatively expensive heat transfer liquid to the atmosphere, while in vapor form in an open top vessel, is the subject of U.S. Pat. No. 3,904,102 of T. Y. Chu et al, issued Sept. 9, 1975, also assigned to the assignee of the present invention. In accordance with the technique disclosed in the last-mentioned reference, a secondary body of vapor, generated by boiling a relatively inexpensive heat transfer liquid, is interposed between the relatively expensive primary body of vapor and the atmosphere. This technique substantially reduces loss to the atmopshere of the hot primary body of vapor confined therebelow. Another form of such apparatus is disclosed in U.S. Pat. No. 4,077,467 of D. J. Spigarelli.
Although such a secondary body of vapor has been found to be quite effective in reducing the losses of the expensive primary vapor, portions of both the primary and secondary vapors are nevertheless still lost to the atmosphere across the secondary vapor-air interface. One reason for this is believed to be the disturbance produced at the primary-secondary vapor interface when normally generating the secondary vapor. The dual vapor losses in question are at least substantially further minimized, however, in accordance with a method and apparatus for more effectively maintaining the secondary vapor body, disclosed in U.S. Pat. No. 4,055,217 of T. Y. Chu, also assigned to the assignee of the present invention, as well as in accordance with the specialized apparatus disclosed in the aforementioned patent of Spigarelli.
With respect to all of the aforementioned dual vapor body generating condensation systems, it is appreciated, of course, that the articles to be heated must be passed downwardly through the upper secondary vapor body in order to be immersed in the primary vapor body. This presents no serious problem with respect to many articles, including certain types of printed circuit boards with only printed circuitry thereon, or having components and/or devices mounted thereon which are not adversely affected by the elevated temperatures of the primary vapor body, in particular.
In an ever-increasing number of mass soldering circuit board applications today, however, the mounted active and passive electronic devices and/or components, particularly when of the solid state integrated circuit type, cannot be safely subjected to a hot saturated vapor body for even relatively short periods of time, and particularly at the elevated temperatures required for soldering. In such cases, and with particular reference to circuit boards, with components mounted on only one side, it would be very desirous to controllably expose only the non-component, printed circuit side thereof to be soldered (hereinafter referred to simply as the underside) to a single hot, saturated (primary) vapor body confined within a vessel, i.e., with no immersion of the completely assembled circuit board within the vapor body. In order to obviate the need of circuit board immersion, of course, the upper boundary, or elevation of the generated body of hot vapor must be well defined.
Such a controlled vapor exposure technique would also be of considerable advantage in the repair of circuit boards, wherein both unsoldering and resoldering operations are normally involved. In this regard, it would likewise be very beneficial if only selected discrete areas on the underside of the circuit board would have to be subjected to the heat of vaporization of a generated body of vapor while, at the same time, minimizing the loss of any vapor to the atmosphere in the absence of an overlying secondary vapor blanket.
One technique employed heretofore to heat only the underside of a printed circuit board in a vapor condensation apparatus has involved a vessel which incorporates an internally cooled and retractable cover plate, the latter being adapted to minimize loss to the atmosphere of vapor generated within the vessel, while allowing the vapor to rise to the top thereof so as to perform, for example, a soldering or unsoldering operation on only the underside of a top side vessel-supported article (e.g., a circuit board). Such an apparatus is disclosed in U.S. Pat. No. 4,194,297, of R. C. Pfahl, Jr., issued Mar. 25, 1980, also assigned to the assignee of the present invention. The internally cooled and retractable cover plate utilized in that apparatus is dimensioned and mounted on the vessel at an elevation such that while in a first extended position, it substantially enclosess the top of the vessel to prevent loss of vapor to the atmosphere, and to also isolate the underside of an article from the vapor when vessel-supported above the cover plate. With the cover plate in a second retracted position, the underside of the supported article is exposed to and heated by the vapor, with the underside of the article then also functioning to enclose the otherwise open top of the vessel. By also preferably tilting the cover plate at a slight angle relative to the underside of the article, any hot vapor that may become entrapped therebetween, prior to the removal of the heated article from the vessel, is substantially completely condensed, with the cover plate allowing the condensate to flow by gravity back to the remaining nonvaporized heat transfer liquid therebelow.
An article entitled "Solvent Vapor Solder Reflow", by E. G. Dingman, IBM Technical Disclosure Bulletin, Vol. 13, No. 3, dated Aug. 1970, describes the use of a boiling solvent (such as that sold under the tradename "Freon E5", by E. I. DuPont de Nemours and Company) to facilitate the removal and resolder of electronic components during printed circuit board rework operations. It is stated therein that "The solvent condenses only on the areas having a temperature lower than the boiling point of the solvent used. This releases the heat of vaporization and enables solder rework operations with materials and components that are heat sensitive. The rapid and selective application of heat to small areas with high thermal conductivity is possible within a matrix of material such is heat sensitive and cannot tolerate high temperatures." While this disclosure discusses the rapid and selective application of heat to small areas of high thermal conductivity, such as the metallic pads, land areas, lead ends and circuit paths of printed circuits boards, there is no suggestion of how to controllably expose a hot saturated vapor body either to only one surface of a printed board having both low and high thermal conductivity areas thereon or, alternatively, to only selective discrete regions encompassed within the areas of high thermal conductivity. Moreover, no physical structure is either illustrated, or described, for accomplishing even the described mode of operation and, particularly, in relation to simultaneously preventing or minimizing loss of vapor to the atmosphere.
It was further appreciated heretofore that other less analogous prior art apparatus also existed of the aforementioned type that requires the confinement of an article within an enclosed vapor generating vessel incorporating some form of cooled top or sidewalls. For example, B. Juettner U.S. Pat. No. 2,716,348 discloses a vessel with a horizontally disposed, water-cooled, top-enclosing cover (removable but not retractable). K. A. Holm et al. U.S. Pat. No. 3,479,252 discloses a sectioned vessel with a removable top portion having water-cooled sidewalls and an air-flow, channel-defining top wall. B. C. Feng U.S. Pat. No. 4,022,932, discloses a completely enclosed and non-cooled vessel, utilizing a simple detachable cover, for making patterned resist masks.
From the foregoing, and with particular reference to vapor condensation apparatus, it is seen that a number of different approaches have been taken with respect to heating either the entire article, or a selected surface area thereof, while simultaneously providing means to minimize the loss of the generated vapor to the atmosphere. While the above-described vessel utilizing a retractable, internally cooled cover plate provides a rather effective way of providing selective underside article surface heating with a minimal loss of vapor to the atmosphere, it would nevertheless be very desirable if the upper boundary of the generated body of vapor could remain relatively stationary at all times within the vessel, in the absence of any type of vessel cover plate, or a secondary vapor blanket, and regardless of whether or not an article to be heated was positioned on the vessel, and/or was selectively exposed to the body of vapor.
Such an upper boundary confined vapor body would, for all practical purposes, obviate the problem of any non-condensed vapor, such as entrapped between a vessel-mounted article and an underlying retractable (or otherwise removable) cover plate, from escaping to the atmosphere. Of course, any attempt to utilize a secondary vapor blanket, as employed in several of the aforementioned prior art apparatus, rather than a vessel-enclosing cover plate, to minimize vapor loss, would not only necessitate the immersion of the entire article to be heated within the secondary vapor, but also within the primary vapor of much higher temperature. This follows from the fact that any attempt to lower an article down through a secondary vapor blanket to an elevation such that only the desired underside surface thereof to be heated is located at the precise primary and secondary vapor interface, would be extremely difficult to achieve, if not impossible. The same problem would arise, of course, with a conventional vapor barrier of substantially uniform volume established above a single primary generated body of hot vapor.